Modern vapor generators or evaporators, such as steam generators or boilers, frequently vaporize a media fluid or coolant by passing it in heat transfer contact over a surface heated to a temperature well above the vaporization temperature of the fluid. The fluid to be vaporized (typically water although hydrocarbons or alcohols are commonly used also) at the pressure of the system is a liquid at "bulk" temperatures below the vaporization temperature, is a vapor at bulk temperatures well above the vaporization temperature, and is at multiphase (liquid-vapor) mixtures at bulk temperatures near the vaporization temperature. The surface can be heated electrically; or more commonly by another "heat-in" fluid, such as hot flue gas circulated along a thermally conductive wall on the side opposite from the heated surface and isolated from the vaporizing or "heat-out" fluid (the reference to heat-in and heat-out fluids is thus made in this disclosure relative to bringing heat into or remove heat out of the vapor generator). One such heat exchanger configuration might be of the shell and tube type where the vaporized heat-out fluid (water for example) flows through tubes running through a larger shell, and the heat-in fluid (flue gas for example) flows through the shell and over the tubes. The flow paths for the two media fluids could of course be reversed. Alternatively instead of using hot flue gas as the heat-in fluids, a hot "coolant" (such as water, a liquid metal like sodium, or a gas such as helium) can be circulated through the heat exchanger configuration, the coolant being heated beforehand by any known means immaterial to this invention.
The design chosen for and the effectiveness of heat transfer between the two media fluids would depend on many factors. These would include the differences in the enthalpy "H" and entropy "S" of the respective media at its inlet and outlet to the vapor generator; the temperature driving force and film coefficients of heat transfer across the fluid-solid interface; the pressures, mass and velocity flow rates of each media in the system; and the cost and safety considerations for providing all of the above.
FIG. 1 illustrates a typical pool-boiling curve commonly associated with multiphase fluid of the type to be vaporized; where the log values of "heat flux" inputs are listed as the vertical axis and the log values of "temperature differences" are listed as the horizontal axis. The "heat flux" is related to the temperature driving force of the heated surface, and will vary between less than 1000 Btu/hr. ft..sup.2 for a low output steam generator and perhaps 250,000 Btu/hr. ft..sup.2 for a very high output commercial steam generator. The "temperature difference" is that between the "heated surface" temperature and the bulk temperature of the liquid or of the steam. Since these values are on log scales in FIG. 1, the curve is quite compact insofar as absolute values are concerned.
As identified generally along the curve between A and B, the fluid to be vaporized is in the liquid phase only and no boiling at all takes place on the surface: this is the "feedwater" phase of heating. From B through S and up to C, the local heat flux is sufficient to raise the water temperature adjacent the heated surface to saturation temperature, or slightly above, and a change from the liquid to the vapor state occurs locally. This change is characterized by the coexistence of both phases at essentially the same temperature locally, differing only in a few degrees of vapor superheat necessary for heat transfer and by heat absorption required to overcome the molecular binding forces of the liquid phase. Here, the change of state is accompanied by ebullition of the vapor at the solid-liquid interface (as opposed to evaporation at a free surface); this is the "nucleate boiling" phase of heating.
The bulk of the liquid does not reach saturation temperature until the heat flux of point S is reached. Between B and S, the vapor bubbles formed at the heated surface condense quickly in the liquid giving up latent heat to raise the temperature of the liquid. This condition is known as subcooled-nucleate or local boiling. Nucleate boiling occurs at all points up to C; beyond S, the bubbles do not collapse, since this part of the curve represents boiling with the liquid bulk temperature at saturation.
Both nucleate-boiling regimes (subcooled and saturated), are characterized by very high heat transfer coefficients. These are ascribed to the high secondary velocities of the liquid caused by the liberation of surface tension energies available in the liquid-vapor-solid interfaces at the instant of bubble release from the heated surface. Thus a convection-type transfer coefficient based on bubble kinetics is also affected to some extent by bulk mass velocity, depending on the velocity range.
Beyond the nucleate boiling region (B-C in FIG. 1), the bubbles of vapor forming on the heated surface begin to interfere with the flow of liquid to the surface and eventually coalesce to form a film of superheated vapor over part or all of the heated surface. This condition is known as "film boiling". From D to E film boiling is unstable; beyond point E film boiling becomes stable.
In heat exchangers where the heat flux exceeds that corresponding to point D, the temperature of the heated surface can rise very quickly, along the horizontal dotted line in the figure to point D'. If the temperature at D' is sufficiently high, the heated surface can burn out or melt. Hence, D is known as the burnout point. C is known as the point of departure from nucleate boiling (DNB), or the critical heat flux.
Stable and even unstable film boiling is regularly encountered in certain types of heat transfer equipment where the temperature of the heat source is within the safe operating range of the equipment, or where the boiling film heat transfer coefficient is the controlling resistance to heat flux. Steam generators for pressurized-water nuclear reactor systems, which are actually water-to-boiling water heat exchangers, and certain types of process heat exchange equipment are in this category.
Depending on the "heat flux", nucleate boiling can start with as low as 0-10% vapor in the combined liquid-vapor mixture and continue then into the higher quality multiphase conditions of the fluid, commonly up to no more than 10-50% vapor for commercial boilers, but even up to almost 100% vapor for very low heat fluxes. However, at very high heat fluxes, and at higher quality vapor (10-50%) and moderately high heat fluxes, so many bubbles are being formed that they in effect blanket the remaining liquid from contacting the heated surface. Under such circumstances, the temperature difference (between the heated surface and the bulk or surface temperature of the fluid) can rapidly increase from D to D', or from D to anywhere on the film boiling curve between E and D'.
The departure from nucleate boiling (DNB) can be an unstable transition condition, since once DNB occurs for any given heat flux, the heat flux that can be used effectively to vaporize the fluid drops off dramatically and any higher heat flux just creates the large "temperature differences" along a curve between D and the film boiling curve between E and D'.
The pool-boiling curve (illustrated in FIG. 1) is representative only of test conditions without forced fluid flow or other agitation that would increase the effective heat transfer coefficients; and thus it is not followed explicitly in any normal vapor generator or steam boiler. However, virtually all multiphase boiler or evaporator sections have different areas or regions where the heating phenomenon converts between "feedwater," "nucleate," and "film" heating. In reference to a steam generator, the feedwater section primarily has only liquid present and operates in the feedwater and/or nucleate heating phases; the boiler section generally has the multiphase liquid-vapor mixture present and operates in the nucleate and film heating phases, and the superheat section has pure vapor present and operates exclusively in the convective phase of heating.
With reference again to FIG. 1, the temperature differences between the heated surface and the bulk fluid at C and D represent perhaps 20.degree.-50.degree. F.; where at D' the corresponding temperature differences is much larger, perhaps even 500.degree.-1000.degree. F. One basic undesirable characteristic of the "departure from nucleate boiling" (DNB) is that this large temperature shift (up to even 1000.degree. F.) can occur quite rapidly (on the order of only seconds). Thus, serious cyclical thermal stress problems can develop to weaken the feedwater and/or boiler sections of the vapor generator where any DNB might actually occur.
It should be noted further that the heat transfer coefficients in the feedwater or nucleate heating phases are very high (typically 1000-2000 for feedwater heating and 2000-100,000 for nucleate boiling); whereas the heat transfer coefficients in the stable film boiling or conductive heating phase are lower (perhaps 200-1000); and the heat transfer coefficients are the lowest (perhaps 100-500) during the DNB or transition phase of heating. These comparative heating rates can be noted generally by the changing steepness of the curve illustrated in FIG. 1, feedwater heating (A-B) being higher than for the stable film boiling (E, D', F), while the nucleate boiling (B, S, C, D) is the highest.
It therefore is advantageous to utilize the feedwater heating and/or nucleate boiling regions as much as possible, as contrasted against the film boiling region, so as to minimize the size of the vapor generator.
FIG. 2 illustrates a known complex vapor generator system that eliminates entirely the DNB region, but does so by having two separate boiler or heater sections separated by a liquid-vapor separator or steam drum. The heat-in fluid is shown as passing sequentially through the superheater and the feedwater heater. The vaporizing heat-out fluid in turn passes as a liquid through the feedwater heater, where it is heated to temperatures equal to or near its vaporization temperature, and subsequently enters the liquid-vapor separator as low quality vapor. Vapor from the liquid-vapor separator is then passed through the superheater and is discharged from the system as high pressure superheated vapor. Liquid from the liquid-vapor separator is drawn off and combined with the feedwater for recirculation through the feedwater heater/boiler. The major drawbacks to this complex system are (1) the need for the different components including the liquid-vapor separator, the feedwater heater/boiler, and the superheater; and (2) the recirculation of a great percentage (300-400%) of liquid from the liquid-vapor separator back through the feedwater heater/boiler, increasing the size and both initial and operational costs of the feedwater heater/boiler and the feedwater pumping equipment.
A second known vapor generator system (the Loffler cycle) is illustrated in FIG. 3 and utilizes only a superheater having sufficient volumetric output of vapor that a large percentage of it (approximately 75%) can be bled off, compressed and then be added as superheated vapor to the liquid feedwater before the latter normally would enter into the superheater. This superheated vapor when combined with the feedwater effectively provides at the inlet to the vapor superheater, a mutliphase mixture at sufficiently high quality (95-100% vapor) that it is beyond where DNB might occur. Thus, the DNB problem is eliminated; but this means that all heat addition in the superheater is to vapor only and is by forced convection only. This coupled with the reduced thermal efficiency of heating vapor only necessitates that the single superheater must be substantially larger than the combined feedwater heater/boiler and superheater generators used in the complex system of FIG. 2; while further the large percentage (75%) of fluid recirculation requires that the superheater be even larger. Also, greater pumping costs are incurred (both in pumps and energy) in recirculating the low density superheated vapor back to the superheater.
In general, two factors limit the output of a vapor generator: one being structurally related and the other being heat flux related. With the structurally related limitation, such as an electric resistance heater, lack of coolant flow and the resultant cooling of the heater could allow the heater to exceed the heated surface upper temperature limits so as to destroy itself. The cooling capacity of the vaporizing fluid need of course be sufficient to cool the surface to within the temperature limits. This then brings into play the second type of limiting design, or the heat flux related factor. If the heated surface itself under normal operating outputs cannot be cooled, the heat flux can be too high relative to the coolant used where overheating would occur notwithstanding the existence or loss of coolant. Under such circumstances, again, the increased temperature of the heated surface could possibly destroy the heated surface itself. These factors, however, do not directly bear on the particular invention, but merely serve to emphasize the significant consequence that can occur under DNB transient conditions with the reduced effectiveness of heat transfer and the consequent temperature increase.